The invention relates to a device for optically scanning an information plane having tracks, the device comprising a radiation source for supplying a radiation beam, means for focusing the radiation beam to a scanning spot on the information plane, a dividing element comprising a first and second area at both sides of a dividing line and arranged in the optical path of radiation from the information plane for forming a first and a second detection beam from the first and second area respectively, a first detector arranged in the optical path of the first detection beam, a second detector arranged in the optical path of the second detection beam, and an electrical circuit for combining output signals of the detectors to produce a focus error signal. The focus error signal may be used in a focus servo loop which controls the longitudinal position of the scanning spot in order to keep the scanning spot in the information plane.
A device of this type which can be used, inter alia, in apparatuses for writing and reading information in the tracks of optical record carriers and is known from U.S. Pat. No. 5,243,585. In the third embodiment described in this document the focus error signal is formed by a split detector irradiated by one beam. The focus error signal is formed as the difference signal of the output signals of the two detectors divided by the sum signal of the two detector signals. A drawback of the known device is that the focus error signal is relatively strongly affected by manufacturing tolerances in the position of the detectors with respect to the detector beams. The above citation is hereby incorporated in whole by reference.
The present invention has for an object to provide a scanning device in which the focus error signal is less sensitive to the position of the detectors.
To this end the scanning device according to the invention is characterized in that the electrical circuit is arranged to form the difference signal of the output signal of the first detector and the second detector and dividing the difference by the sum signal of said two output signals. A shift of the position of the combination of the two detectors in the XY plane changes the amount of radiation intercepted by each detector in the same way. The focus error signal formed according to the invention takes this fact into account, and is therefore less sensitive to a shift of the detectors from the intended position compared to the known scanning device. The invention especially reduces the influence of the position of the combination of the detectors on the slope of the focus error signal as a function of the longitudinal position of the scanning spot. This reduction stabilizes the focus servo loop.
A preferred embodiment of the device according to the invention is characterized in that the first detector and a third detector are disposed on two sides of a first line, and the second detector and a fourth detector are disposed on two sides of a second line, the first and second line being substantially parallel to the dividing line, the circuit forming a focus error signal proportional to (Saxe2x88x92Sb)/(Sa+Sb) and to (Sdxe2x88x92Sc)/(Sa+Sc), in which Sa, Sb, Sc and Sd are the output signals of the first, second, third and fourth detector respectively. The combination of the two difference signals provides a focus error signal which is tolerant for both position errors of the detectors and the position of the dividing line in the reflected beam. Moreover, the combination doubles the focus error signal as compared to the single difference signal. Lines are substantially parallel when the angle between the lines is smaller than about 30xc2x0.
A further embodiment of the device according to the invention, allowing push-pull radial tracking and providing a low radial-to-focus crosstalk is characterized in that the areas are located symmetrically at both sides of a dividing line of the dividing element, the angle between the dividing line and an effective direction of the tracks being between 15xc2x0 and 80xc2x0. Radial-to-focus crosstalk is the deterioration of the focus error signal when scanning the spot over the information plane in a direction perpendicular to the tracks.
A special embodiment of the device according to the invention is characterized in that the dividing element comprises a diffraction grating and each area comprises a sub-grating. A grating is a relatively cheap element performing the two functions of redirecting part of the radiation from the record carrier to the detection system and splitting the reflected radiation into two detection beams.